The present invention relates generally to composite materials containing fibers and a carbon matrix, and more particularly to a process for fabricating thin carbon-fiber carbon-matrix composites of high density.
Carbon fiber-carbon matrix (C--C) composites are a class of lightweight, very high-temperature materials that have a variety of niche applications in the aircraft and aerospace industries. Among these applications, use in the fabrication of rocket nozzles, reentry nosetips and heat shields, and aircraft brake disks are some of the most important.
C--C composites are fabricated in general by a two-step method in which a carbon fiber preform is first assembled by a variety of textile processes that either directly yield useful shapes or produce bulk materials that are subsequently shaped. Second, the carbon matrix is formed by depositing the matrix into the preform by either repeated cycles of liquid impregnation and pyrolysis, or continuous processing by simultaneous gas infiltration and pyrolysis. During the liquid infiltration method, fluid carbonaceous resins or pitches are soaked into the porous fiber preform and then are pyrolyzed, or thermally decomposed, to form carbon. In contrast, the gas infiltration method utilizes a flowing hydrocarbon gas that is allowed to diffuse into the fiber preform under conditions in which the gas slowly decomposes to form carbon.
Several references are illustrative of this basic processing technology. For instance, U.S. Pat. No. 3,174,895, Gibson, et al., discloses a method of fabricating artificial carbon or graphite bonded cloth laminates that have flexibility, strength, and electrical property advantages over monolithic artificial graphites. Graphite cloth sheets are painted with a carbonaceous binder, stacked, molded, cured together under pressure and baked to form the laminates. U.S. Pat. Nos. 3,233,014 and 3,238,054, Bickerdike, et al., disclose a method for fabricating fibrous carbon articles by pyrolytic deposition in which a fibrous preform is heated in a stream of gas containing a gaseous carbon compound so that carbon is decomposed from the gas to deposit within the preform to form the matrix; alternatively, the carbon matrix can be formed by a process in which a synthetic carbon-yielding resin, such as a furfuryl alcohol, impregnates the preform and the resin is subsequently polymerized and carbonized. U.S. Pat. No. 3,462,289, Rohl and Robinson, teaches a method of producing high density reinforced carbon and graphite bodies whereby a carbon or graphite fiber preform is made into a shape and pressure impregnated in vacuum by a suitable carbonaceous resin, followed by pressure curing and baking. The entire impregnation and baking cycle must be repeated in order to properly densify the part. The number of cycles is determined by the density that is desired in the finished shape. It is believed that these patents define the current state-of-the-art in C--C composite processing technology.
An important and fundamental drawback with these processing methods serves to increase the cost and complexity of C--C composites to the point that C--C composites can only be used in highly specialized applications. Because C--C composites begin to oxidize at temperatures as low as 425.degree. C., all current processing methods require prolonged and/or repeated processing at high temperatures in the absence of oxygen. Although many modifications have been made in the past decades to streamline the C--C composite fabrication process, none of these modifications represent more than incremental improvements to these basic methodologies.